Optical resonators include two or more mirror structures that define the resonator cavity. Optical resonators can be passive cavity devices as used, for example, in tunable Fabry-Perot (FP) filters. Active cavity devices also include a gain medium, such as a semiconductor or a solid-state material, inside the cavity between the mirror structures. The most common example of an active cavity optical resonator is the laser.
A reoccurring issue in optical resonator design, both in macroscopic and micro optical systems, is transverse spatial mode control. At scales associated with micro optical systems, which include single mode optical fiber, semiconductor gain media, and/or micro-optoelectromechanical system (MOEMS) devices, spatial mode control can dictate many system design variables.
Typically, fundamental transverse mode operation is desired in laser devices because of the optical beam spatial profile requirements for long distance beam propagation, focusing of beams into small spots, and beam coupling into single mode transmission fibers. In addition, different spatial modes of an optical resonator typically have different resonant optical frequencies, which characteristic is detrimental for active and passive cavity applications requiring spectral purity. A typical application requiring spectral purity of the resonator operation is optical spectral monitoring using tunable Fabry-Perot filters of optical signals such as wavelength-division-multiplexed (WDM) optical communication signals.
In active cavity devices, such as edge-emitting semiconductor lasers, the transverse spatial mode problem is addressed by the judicious design of the laser waveguide to ensure that it supports only a single transverse mode. In vertical cavity surface emitting lasers (VCSEL""s), oxide confining layers and other aperturing techniques are used to achieve single transverse mode operation in small aperture devices. Problems begin to arise as higher output power devices are designed, however. There is contention between the desire to increase modal volume and beam diameter while at the same time suppressing oscillation of the higher-order transverse modes.
In passive cavity devices, the transverse mode problems can be more intractable, since the design degree of freedom associated with the gain medium is not present. One solution is to incorporate single mode fiber into the design. The inclusion of fiber, however, tends to complicate increased device integration, creates fiber-coupling requirements, and does not resolve all of the spatial mode problems. For example, a detector can be responsive to the input TEM20 mode even with spatial mode filtering offered by a single mode fiber; this is due to the substantial amount of power propagating in the leaky and cladding modes of the fiber.
A related solution to controlling the transverse side mode suppression ratio (SMSR) contemplates the use of intracavity apertures or spatial filters. Higher order spatial modes generally have larger mode field diameters than the fundamental TEM00 mode. As a result, apertures in the optical train can induce loss for higher order transverse modes and may be used to improve the side mode suppression. These spatial filters, however, can introduce some loss to the fundamental mode as well as to the higher order modes; they also require precise alignment.
Still another solution concerns cavity design. In a confocal Fabry-Perot cavity, where cavity length is equal to the mirror radius of curvature, all transverse modes are degenerate, i.e., all the transverse modes coexist on the same frequencies, or wavelengths, as the longitudinal mode frequencies or the longitudinal mode frequencies shifted by a half spectral period. MOEMS micro optical cavities typically have large free spectral ranges, or spectral periods, corresponding to small cavity lengths of only tens of micrometers, however. Therefore the confocal MOEMS micro cavity configuration would require mirrors with correspondingly small radii of curvature, i.e., tens of micrometers, which are difficult to fabricate, and have small mode sizes, which are difficult to align.
A more typical configuration for MOEMS tunable filter Fabry-Perot cavity is termed a hemispherical cavity. In such cavities, one of the reflectors is near planar and the other reflector is a spherical reflector. The advantage is reduced alignment criticalities because of the general radial homogenatiy of the flat reflector. In such configurations, spatial mode spectral degeneracy is not present and higher order transverse modes present a problem-spurious peaks are observed in the filter transmission spectrum.
These problems have led to solutions that focus on minimizing the excitation of higher order modes by precise control of how light is launched into the cavity. For example, U.S. patent application Ser. No. 09/666,194, filed on 21 Sep. 2000 by Jeffrey A. Korn, and Ser. No. 09/747,580, filed 22 Dec. 2000 by Walid A. Atia, et al., concern, in part, alignment of a tunable filter relative to the surrounding optical train. U.S. patent application Ser. No. 09/809,667, filed on 15 Mar. 2001 by Jeffrey A. Korn, concerns mode field matching between the launch light mode and the lowest order spatial mode of the filter. Such approaches minimize excitation of higher order spatial modes and thus yield systems with high side mode suppression ratios.
The present invention is directed to the design of the optical resonator cavity mirrors and/or intracavity lenses. The design intention for these mirrors/lenses is to degrade the ability of the resonator to support higher order transverse spatial modes. Higher order transverse modes are forced to be unstable in the inventive optical resonator, ultimately achieving improved transverse mode operation to single transverse mode resonator operation.
Generally, previous approaches to transverse mode control in optical resonators spatially varied only the magnitude of optical beam transmission or reflection inside the resonator in order to introduce differential loss for higher order stable transverse modes. In contrast, the present invention includes spatially varying the phase of optical beam transmission or reflection in the resonator in order to make higher order transverse modes of the resonator fundamentally unstable or unbound.
The invention can be analogized to optical fibers, which achieve single transverse mode operation by using spatially varying refractive indices, and hence spatially varying optical phase delay, to make higher order transverse modes of the fiber unbound and hence to suppress their propagation. Unfolded optical resonators are represented by discrete lens waveguides, with certain analogies to the continuously guiding waveguides, such as optical fibers. In the present invention, analogously with controlling the fiber refractive index profile, the mirror shape or lens profile is tailored to produce the desired spatial distribution of the intracavity optical phase delay, which selectively suppresses resonance of higher order transverse modes in the cavity. As a result, single transverse mode operation of optical resonators is achieved.
The path to implementing the invention is in the context of micro optical systems, which include single mode optical fiber, semiconductor gain media, and/or MOEMS devices. In such systems, the maximum beam diameters are typically less than a few millimeters. Specifically, in the present implementations, the maximum beam diameters in the system are less than 500 micrometers (xcexcm). Small beams are consistent with the small form factors required in most optical communications applications, and also reduce the beam diameter translation requirements when coupling into and out of single mode fibers, which typically have a 5 to 10 xcexcm mode diameter. Further, the beams as launched into the optical cavities are small, typically less than 200 xcexcm. In current implementations, the launched beams are actually considerably smaller, less than 50 xcexcm, or between 5 and 30 xcexcm.
In more detail, according to one aspect, the invention features a passive optical resonator comprising at least one optical cavity defined by at least two mirror structures in which at least one of the mirror structures has a mirror profile having a diameter and sag that are selected in combination with a length of the cavity to degrade a stability of transverse modes with mode numbers 4 and greater.
Generally, in such implementations, the optical cavity length is less than about 50 micrometers, a sag of the optical surface is less than about 200 nanometers, and the full width half maximum of the mirror diameter is less than about 30 micrometers. More commonly, the optical cavity length is less than about 30 micrometers, a sag of the optical surface is less than about 150 nanometers, and the full width half maximum of the mirror diameter is less than about 20 micrometers. As described in more detail below, optical cavity lengths of less than about 20 micrometers, with optical surface sags of less than about 100 nanometers, and the full width half maximum mirror diameters of less than 15 micrometers have been produced. In these cases, the sag is an important parameter, since the relationship between the mirror sag and the effective mode deflections for the higher order transverse modes leads to the inability of the cavity to support these modes, according to one line of analysis.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.